Vehicles typically include an exhaust gas recirculation (EGR) system to selectively direct internal combustion engine exhaust gas to an air inlet of the engine. EGR can lower the level of certain undesirable engine emission components such as nitrogen oxide (NOx) and can improve fuel economy. Up to a limit, NOx emissions decrease with increasing EGR levels. Beyond the limit, EGR can increase formation of other undesirable engine emission components and can reduce vehicle drivability.
EGR typically involves recirculation of exhaust gas through an EGR passage between an engine exhaust conduit and an engine fresh air intake passage. A valve within the EGR passage (the EGR valve) is controlled to vary a restriction within the EGR passage to regulate the flow of exhaust gas therethrough. In compression ignition engines, recirculated exhaust gas may be cooled to enable induction of a greater mass of exhaust gas into the engine cylinders.